It has been reported by the Colorado School of Mines that the inorganic portion of Green River oil shales contains around 15 weight per cent calcite (CaCO.sub.3) and about 35 weight per cent dolomite (CaCO.sub.3.MgCO.sub.3). Decomposition of the carbonates begins at around 1300.degree.F and proceeds rapidly above 1600.degree.F. Care must be taken to control the amount of carbonate decomposition during spent shale combustion, since this reaction is highly endothermic and will negate the exothermic heat of combustion of the carbon on the spent shale.
Green River oil shale actually is not a shale but is a marlstone with the kerogen acting as a binder to hold together the finely divided inorganic particles. The kerogen is decomposed during pyrolysis and the spent shale produced has little compressive strength, tending to disintegrate when minor forces are applied. Combustion of the carbon on the spent shale removes the remaining binder and, coupled with carbonate decomposition, the resulting shale ash is extremely friable, being produced predominantly as a powder-like residue.
Several processes, such as those disclosed by U.S. Pat. Nos. 2,480,670 and 3,597,347 propose that the shale ash be recycled to the pyrolysis vessel to provide the heat required for retorting. Other processes, such as disclosed by U.S. Pat. Nos. 2,983,653 and 3,655,518, propose that a heat carrier together with a portion of the shale ash be recycled to the pyrolysis vessel to provide the heat required for retorting. Control of the flow of such finely divided material and the potential of contamination of the oil product with bottoms sediment resulting from the entrainment of shale ash into the pyrolysis vapors are two of the major problems faced by such processes.
U.S. Pat. Nos. 2,634,233 and 3,691,056 point out another problem of the adsorption of pyrolysis vapors on the shale ash. The latter patent discloses a method whereby heat recovery is accomplished while substantially reducing the amount of shale ash circulating to the pyrolysis vessel.